UPPER THERMAL TOLERANCES OF 12 NEW-ZEALAND STREAM INVERTEBRATE SPECIES

The upper thermal tolerances of 12 New Zealand freshwater invertebrate taxa were compared using a laboratory lethality testing protocol. Tem peratures that were lethal to 50% of the test organisms (LT(50)s), fol lowing acclimation to 15 degrees C, generally declined over the 4 day period of the tests. LT(50) values after 48 and 96 h exposure ranged f rom 24.5 to > 34 degrees C and 22.6 to 32.6 degrees C, respectively, i ndicating that temperatures that occur in summer in many NZ streams an d rivers may limit the distribution and abundance of some of these inv ertebrate species. Larval insects included both the most sensitive spe cies (a plecopteran and two Ephemeroptera) and the most tolerant (the larvae of the elmid beetle Hydora sp.) in this study. The two molluscs studied (Potamopyrgus antipodarum (Prosobranchia: Hydrobiidae) and Sp haerium novaezelandiae (Bivalvia: Sphaeriidae)) showed high thermal to lerance, whilst two crustaceans (Paratya curvirostris (Atyidae) and Pa racalliope fluviatilis (Eusiridae)) were moderately tolerant. The effe ct of a higher acclimation temperature (20 degrees C) on thermal toler ance was also investigated for the three most sensitive species. This appeared to increase the thermal tolerance of the two Ephemeroptera (D eleatidium spp, and Zephlebia dentata (both Leptophlebiidae)) but to r educe the tolerance of the plecopteran Zelandobius furcillatus (Gripop terygidae).